1. Field of the Invention
The present invention relates to a method of and an apparatus for drying a wafer. More particularly, the present invention relates to a method of and an apparatus for drying a wafer using isopropyl liquid after the wafer is cleaned following a semiconductor manufacturing process.
2. Description of the Related Art
Semiconductor devices or semiconductor chips are manufactured by processing a wafer that is usually formed of silicon. The wafer is typically subjected to a series of semiconductor device manufacturing processes such as photolithography, chemical or physical vapor deposition and plasma etching.
After executing these processes, foreign material such as chemicals or dust remains on the surface of the wafer. In order to assure the quality of the semiconductor devices, the foreign material must be removed from the surface of the wafer. The cleaning process used to remove the foreign material involves both washing and drying the wafer.
In particular, the wafer may be washed using de-ionized water (hereinafter referred to as xe2x80x9cDIWxe2x80x9d). Because the DIW will eventually dissolve the silicon, the wafer must be completely dried after being washed with the DIW or else water spots will be formed.
In addition, a method of using isopropyl alcohol to enhance the drying of a wafer has been developed. For example, Japanese Patent Laid-Open Publication No. Hei 8-61846 discloses a method of directly spraying liquid isopropyl alcohol over a wafer, forming a mixture of the water and the isopropyl alcohol on the surface of the wafer, and volatilizing the mixture by means of a high temperature nitrogen gas. In addition, U.S. Pat. No. 6,029,371 issued to Kamikawa et al. discloses a method of drying a wafer by directly spraying a washed wafer with a drying gas comprising heated isopropyl alcohol and nitrogen.
Also, drying apparatuses using isopropyl alcohol are disclosed in U.S. Pat. No. 5,634,978 issued to Mohindra et al., U.S. Pat. No. 5,855,077 issued to Chang-Hyun Nam et al., U.S. Pat. No. 4,633,893 issued to McConnell et al., and U.S. Pat. No. 4,911,761 also issued to McConnell et al. These drying apparatuses execute a method in which the isopropyl alcohol is introduced as a mist over the cleaned wafer to eliminate the water on the wafer.
FIG. 1 shows one example of a conventional apparatus 100 that works on the Marangoni effect to dry the wafer using an isopropyl alcohol mist. The apparatus 100 includes a cleaning section (or a rinsing section) 110 containing a cleaning liquid (or rinsing liquid) 113 for cleaning (or rinsing) the wafer. The cleaning section 110 in turn includes an inner bath 112 having an upper open end, and an outer bath 116 covered with a lid 117. The outer bath 116 and lid 117 enclose the inner bath 112. A wafer 101 is immersed in the cleaning liquid 113 of the inner bath 112 for cleaning. Once the cleaning liquid 113 over-flows the inner bath 112, the over-flown cleaning liquid 113a gathers in the outer bath 116 from where it is drained from the cleaning section 110.
A cleaning liquid supply tube line 114a is connected to the inner bath 112 at the bottom thereof. First and second cleaning liquid drain tube lines 114b and 114c are connected to the inner bath 112 and the outer bath 116, respectively, at the bottoms thereof. The cleaning liquid supply tube line 114a supplies the cleaning liquid 113, such as DIW, to the inner bath 112. The first cleaning liquid drain tube line 114b drains the cleaning liquid 113a which has over-flown the inner bath 112 into the outer bath 116. The second cleaning liquid drain tube line 114c gradually drains the cleaning liquid 113 from within the inner bath 112.
The apparatus 100 for drying the wafer is also equipped with an isopropyl alcohol mist supply tube line 134 for supplying the isopropyl alcohol mist from an isopropyl alcohol supply unit (not shown). The isopropyl alcohol supply unit makes the isopropyl alcohol bubble by using nitrogen as a carrier gas to form the isopropyl alcohol mist. Then, the isopropyl alcohol mist and nitrogen are supplied to the upper portion of the outer bath 116 via the isopropyl alcohol mist supply tube line 134. A diffuser 136 is furnished at the central portion of the lid 117 of the outer bath 116 for consistently diffusing the isopropyl alcohol mist and nitrogen throughout the inside of the outer bath 116.
In addition, a nitrogen gas supply tube line 140 is connected to the lid 117 for supplying heated nitrogen gas into the outer bath 116 during the drying process to create a drying ambient.
The conventional apparatus 100 for drying a wafer operates as follows.
Once a wafer guide 103 loaded with wafers 101 is seated within the inner bath 112, the cleaning liquid 113, such as the DIW, is supplied to the inner bath 112 via the cleaning liquid supply tube line 114a to initiate the cleaning operation. The cleaning liquid is supplied into the inner bath 112 at such a rate that it overflows the inner bath 112 during the cleaning operation. The over-flown cleaning liquid 113a is gathered in the outer bath 116 and drained from the bottom of the outer bath 116 via the first cleaning liquid drain tube line 114b. 
Once the cleaning operation is complete, the process of drying the wafer 101 begins. FIGS. 2A, 2B and 2C are schematic diagrams illustrating the drying process.
Referring to FIG. 2A, after the cleaning process is complete, the nitrogen gas and the isopropyl alcohol mist entrained thereby are introduced via the isopropyl supply tube line 134 and diffuser 136 into the upper portion of the outer bath 116. Thus, the ambient in outer bath 116 is converted into a drying ambient. At this time, approximately 50 cc of the isopropyl alcohol mist is supplied. Also, at this time, the heated nitrogen gas is supplied into the outer bath 116 via the nitrogen gas supply tube line 140 connected to the lid 117.
Referring to FIG. 2B, the cleaning liquid 113 is drained via the drain tube line 114c while the nitrogen gas is supplied via the nitrogen gas supply tube line 140. At this time, the cleaning liquid 113 is drained at a constant rate via the second drain tube line 114c. The height of the cleaning liquid 113 is decreased at a rate of about 3 mm/sec. During the draining operation, the water spots on the wafers 101 are eliminated by means of the Marangoni effect created by the isopropyl alcohol. Referring to FIG. 2C, once the cleaning liquid 113 is completely drained to a level below the wafers 101, the last of the heated nitrogen gas is introduced into the outer bath 116 via the nitrogen supplying tube line 140, thereby completing the drying process.
In addition to this conventional method, Japanese Patent Laid-Open Publication Nos. Hei 11-87305, Hei10-154689 and Hei10-22257 disclose methods of drying a wafer in which an isopropyl alcohol mist is used. In these methods, an isopropyl alcohol liquid layer is formed by the mist over a cleaning liquid, and the wafers are raised from the cleaning liquid into the isopropyl alcohol liquid layer, thereby drying the wafer.
According to all of these heretofore known methods, the mist of isopropyl alcohol is supplied to the outer bath using nitrogen as a carrier gas. Therefore, the amount of isopropyl alcohol injected into the outer bath is determined by measuring the reduction in the amount of isopropyl alcohol in the isopropyl alcohol supply unit. For this reason, the amount of isopropyl alcohol used for forming the isopropyl alcohol layer cannot be accurately determined. Furthermore, it is difficult to accurately control the amount of isopropyl alcohol being supplied.
Moreover, the isopropyl alcohol sprayed by the diffuser adheres to the side wall of the outer bath. In this case, the isopropyl alcohol is likely to fall as drops onto the wafer. These drops create wafer defects.
Additionally, a large amount of time is required to form from the mist a sufficient layer of isopropyl alcohol above the cleaning liquid within the inner bath for producing the Marangoni effect. The overall processing time is thus significant. Moreover, such a mist is likely to be exhausted if the outer bath is not completely sealed.
In view of these problems of the conventional art, a method has been proposed in which the isopropyl alcohol liquid is supplied directly onto the upper portion of the cleaning liquid. For example, Japanese Patent Laid-open No. Hei 9-213672 discloses a method in which an isopropyl alcohol layer is formed over a cleaning liquid in which a wafer is immersed, and the wafer is dried while ascending from the cleaning liquid. In this method, the isopropyl alcohol layer formed on the cleaning liquid has a thickness of about 5 mm, and the wafer is to raised gradually from the cleaning liquid.
However, the above-described method requires a large quantity of isopropyl alcohol and a relatively long amount of time for forming the isopropyl alcohol liquid layer. Also, the drying effect produced by this method is so insufficient that water spots remain after the wafer is cleaned. The water spots leave too many particles on the wafer.
Therefore, the object of the present invention is to overcome the above-described drawbacks of the conventional prior art.
More specifically, a first object of the present invention is to provide a highly effective and time-efficient method of drying a wafer using the Marangoni effect. A second object of the present invention is to provide an apparatus for drying a wafer particularly suitable for performing such a method.
To achieve the first object, the present invention provides a method of drying an object comprising supplying heated inert gas over a cleaning liquid in which the object is immersed, heating a liquid having a smaller surface tension than that of the cleaning liquid, and then supplying the heated liquid onto the cleaning liquid in a fluid state. This forms a liquid layer on the cleaning liquid, and an ambient over the cleaning liquid comprising the vapor of the heated liquid and the inert gas. Then, the object is removed from the cleaning liquid while the heated liquid layer is maintained. The object is dried further by supplying more of the heated inert gas into the ambient surrounding the object.
The heated inert gas is preferably nitrogen gas, and the liquid used to form a layer on the cleaning liquid is preferably isopropyl alcohol.
To achieve the second object, the present invention provides a cleaning apparatus comprising a cleaning section housing the cleaning liquid for cleaning the object, and an isopropyl alcohol supplying unit that supplies heated liquid isopropyl alcohol onto the cleaning liquid to form an isopropyl alcohol liquid layer and an isopropyl alcohol ambient over the cleaning liquid, and a nitrogen supply unit that supplies heated nitrogen gas above the cleaning liquid housed in the cleaning section. A separating unit can remove the object from the cleaning liquid while the isopropyl alcohol layer is maintained.
According to the present invention, to dry the object such as a wafer, the nitrogen gas is heated and supplied into the cleaning section at a location above the cleaning liquid in which the wafer is immersed, and the liquid isopropyl alcohol liquid is heated and supplied directly onto the cleaning liquid. The isopropyl alcohol liquid layer diffuses quickly to form a liquid layer on the cleaning liquid, the liquid isopropyl alcohol liquid supplied in the fluid state also forms an ambient with the nitrogen gas that is non-volatile above the cleaning liquid. The wafer is dried only by removing it from the cleaning liquid into the ambient while only the heated nitrogen gas continues to be supplied.